The present invention relates to a conductive member, a charging member using the conductive member, a process cartridge using the charging member and an image forming apparatus using the process cartridge.
For an image forming apparatus of electrophotographic type such as a copying machine, laser beam printer or facsimile, there has been utilized a conductive member as a charging member, which performs a charging process to a photoreceptor drum (image carrier), or as a transfer member, which performs a transfer process to toner on a photoreceptor drum.
FIG. 1 shows a schematic view of an image forming apparatus. The image forming apparatus 1 comprises a photoreceptor drum 4 as an image carrier in which an electrostatic latent image is formed, a charging roller 2 as a charging member which performs a charging process to the photoreceptor drum 4, a power pack (voltage applying source) 3 which applies voltage to the charging roller 2, a surface potential meter 5 which measures the surface potential of photoreceptor drum 4, a development roller 6 which transfers toner to the electrostatic latent image of the photoreceptor drum 4, a transfer roller 7 which transfers the toner image on the photoreceptor drum 4 to a recording paper S, and a cleaning device 8 which cleans the photoreceptor drum 4 after being transferred. In addition, as shown in FIG. 2, a process cartridge 9 having the photoreceptor drum 4, charging roller 2, development roller 6 and cleaning device 8 may be disposed in the image forming apparatus 1.
The charging roller 2 receives electric supply from the power pack 3 to charge the photoreceptor drum 4 at predetermined potential. The photoreceptor drum 4 rotates in the arrow A direction by means of a driving mechanism (not shown). The surface potential meter 5 is disposed just after the charging roller 2 along the rotation direction so as to measure the potential of a surface 4a of the photoreceptor drum 4.
The development roller 6 transfers the toner to the charged photoreceptor drum 4. The transfer roller 7 transfers the toner, which is transferred to the photoreceptor drum 4, to the recording paper S. The cleaning device 8 eliminates the toner stayed on the photoreceptor drum 4 so as to clean the photoreceptor drum 4.
In an image forming process by the image forming apparatus 1, at first, the surface 4a of photoreceptor drum 4 is charged by the charging roller 2 at negative high potential. Next, the surface 4a is exposed. By this exposure L, each potential on the surface 4a becomes potential distribution according to the amount of light received; thereby, the electrostatic latent image is formed on the surface 4a. 
If a part of the surface 4a on which the electrostatic latent image is formed passes through the development roller 6 by the rotation of photoreceptor drum 4, the toner is transferred to the surface 4a according to the potential distribution of the surface 4a; thereby, the electrostatic latent image is visualized as the toner image. This toner image is transferred to the recording paper S, which is fed at predetermined timing, by the transfer roller 7, and then the recording paper S is fed in the arrow B direction toward a fixing unit (not shown).
On the other hand, after the transfer, the toner stayed on the surface 4a is eliminated by the cleaning device 8 to clean the photoreceptor drum 4, and also the charge is eliminated by a quenching lamp (not shown) to move on to a next image forming process.
As a general charging method in the above image forming apparatus 1, there has been known a contact charging method which brings the charging roller 2 into contact with the photoreceptor drum 4 (for example, reference to JP S63-149668A, JP H01-211779A and JP H01-267667A).
However, the charging roller 2 including the contact charging method has the following problems.    (1) The component of charging roller oozes from the charging roller, and the oozed component is firmly fixed to the surface of photoreceptor drum. If the fixation is developed, the charging roller track stays on the surface of photoreceptor drum.    (2) When applying alternating voltage to the charging roller 2, the charging roller which has contact with the photoreceptor drum vibrates, causing the charging noise.    (3) The toner on the surface of photoreceptor drum is firmly fixed to the charging roller, causing a decrease in the charging performance. Especially, if the component of charging roller oozes as described in (1), the toner is firmly fixed to the charging roller easily.    (4) The component comprising the charging roller is firmly fixed to the photoreceptor easily.    (5) If the photoreceptor is not driven for a long period of time, the charging roller permanently deforms.
In order to handle the above problems, there has been considered a proximity charging method which brings the charging roller 2 close to the photoreceptor drum 4 without bringing the charging roller 2 into contact with the photoreceptor drum 4 (reference to JP H03-240076A, etc.). In this proximity charging method, the charging roller 2 faces the photoreceptor drum 4, such that the distance of closest approach (hereinafter referred to as a space) between the charging roller 2 and the photoreceptor drum 4 becomes 50 μm-300 μm, to charge the photoreceptor drum 4 by applying voltage to the charging roller 2.
Since the charging roller 2 does not have contact with the photoreceptor drum 4 in the proximity charging method, the proximity charging method does not cause “the fixation of the component of charging roller to the photoreceptor” and “the permanent deformation of the charging roller caused by a long period of time nonuse” which are the problems of the contact charging method. In addition, regarding “the decrease in the charging performance of charging roller caused by the fixation of toner”, the proximity charging method is superior to the contact charging method because the amount of toner which is firmly fixed to the charging roller decreases.
In a non-contact charging method, since the charging roller does not have contact with the photoreceptor drum, an elastic body is not required. Therefore, a hardening thermoplastic resin can be used.
Thereby, the space between the photoreceptor and the charging roller can be uniformed. There has been known that the charging mechanism of the surface of photoreceptor drum by the charging roller is discharge according to Paschen's law by micro discharge between the charging roller and the photoreceptor drum. In order to obtain a function which maintains the photoreceptor drum at a predetermined charging potential, it is necessary to control an electric resistance value of thermoplastic resin at a semi-conductive range (about 106 Ωcm-109 Ωcm).
As a method of controlling an electric resistance value, there has been known a method of dispersing a conductive pigment such as a carbon black into a thermoplastic resin. However, if an electric resistance adjusting layer is set to a semi-conductive range by using the conductive pigment, the variations in the electric resistance value increase; thereby, a local charging defect occurs, or local discharge (leakage discharge) occurs due to the electronic conduction, resulting in an image defect.
On the other hand, by using an ionic conductive material as another method of controlling an electric resistance value, the variations in the electric resistance value decrease and also the local leakage which is caused when using the electronic conduction system can be prevented. Accordingly, the charging defect can be prevented.
The ionic conductive material includes an electrolyte salt such as an alkali metal salt or an ammonium salt. However, such a salt is low-molecular weight having a property which easily bleeds out on a surface of matrix resin. As a result, if the bleeding-out occurs on the surface of charging roller, the toner is firmly fixed to the charging roller, causing an image defect. In addition, polarization is caused by the power distribution, so an electric resistance value increases by a temporal change.
Consequently, in order to avoid the bleeding-out, there has been proposed a charging roller using a high-molecular-weight ionic conductive material (JP2005-91818A). In this case, since the ionic conductive material is dispersed and fixed in a matrix resin, the bleeding-out hardly occurs on the surface of charging roller. A polyamide elastomer (PEEA) is mainly used as this type of high-molecular-weight ionic conductive material.
However, even though this type of high-molecular-weight ionic conductive material is used as a conductive member, a target low electric resistance value can not be obtained only with the high-molecular-weight ionic conductive material. In order to solve this problem, there has been considered to add a salt so as to obtain a preferable electric resistance value. In order to lower the electric resistance value, it is necessary for a salt to easily ion-dissociate.
For this purpose, a perchlorate is often used. Consequently, there has been considered to use a sodium perchlorate, a lithium perchlorate or the like. However, in case of using these, when the sodium perchlorate is ion-dissociated, a strong alkaline sodium hydroxide is generated by the reaction with the moisture in the air. Accordingly, the thermoplastic resin is deteriorated by a temporal change, causing a solvent crack.
In addition, there has been known a technique similar to the present invention disclosed in JP2002-311687, which adds an organic phosphonium salt into a thermoplastic resin.